US20090044921A1

US20090044921A1 - Bentonite for binding impurities during paper production

Info

Publication number: US20090044921A1
Application number: US11/721,229
Authority: US
Inventors: Ulrich Sohling; Hubertus Besting; Genovefa Wendrich
Original assignee: Sued Chemie AG
Current assignee: Sued Chemie AG
Priority date: 2004-12-16
Filing date: 2005-11-30
Publication date: 2009-02-19
Also published as: KR20070089805A; EP1825056A2; PL1825056T3; ES2531071T3; JP2008524451A; MX2007006952A; WO2006063682A2; DE102004060587A1; BRPI0515785A; WO2006063682A3; PT1825056E; EP1825056B1

Abstract

A method for binding impurities in paper production, comprising the following steps:

a) provision of a bentonite, the proportion of the monovalent cations, based on the cation exchange capacity (CEC) of the bentonite, being at least 0.7 and the CEC being more than 85 meq/100 g, preferably more than 90 meq/100 g, in particular more than 95 meq/100 g;

b) addition of the bentonite according to a) to a paper pulp or fiber suspension;

c) binding of the impurities to the bentonite in the pulp or fiber suspension

is described.

Description

The present invention relates to the use of special bentonites having a high cation exchange capacity in the binding or removal of impurities in paper production.
The removal or binding of impurities in paper production is becoming increasingly important. The problem is also based on the fact that the water occurring in paper production is circulated, impurities gradually becoming more concentrated therein. These impurities can thus lead to a very wide range of product faults, such as, for example, to the formation of deposits on the rolls of the paper machine, to blocking of the wires by adhesion, etc. These effects lead to interruptions to paper production. In order to minimize the number of production stops, it is desirable to bind the impurities occurring in the circulation water by using polymers or adsorbents in the headbox itself. Most relevant impurities are negatively charged. These are, for example, humic acids, tree resin colloids, lignin derivatives, ligninsulfonates, which are introduced from the fibers into the paper circulation. There are also anionic impurities, which are introduced into the paper machine by recycling of broke. This broke is typically re-dispersed and introduced into the paper machine. As a result, the ingredients and auxiliaries present therein are completely recycled into the circulation. For example, carboxymethylcelluloses, polyacrylates, polyphosphonates and silicates are additionally introduced thereby. Further anionic charged impurities are the latices which are used in the paper coat, which are typically hydrophobic but also carry anionic charges. These have a strong tendency to agglomeration, the agglomerates being deposited as tacky, white residues on the paper machine (so-called white pitch).
The prior art extensively describes the discharge of stickies by the use of talc. Thus, according to P. Biza, E. Gaksch and P. Kaiser “Verbesserter Austrag von Stickys durch den Einsatz von Talkum [Improved discharge of stickies by the use of talc]”, Wochenblatt für Papierfabrikation 11/12 (2002), page 759 et seq., the effect of talc on the reduction of tacky deposits has been documented since the beginning of the last century at the latest. Almost all known natural and synthetic tacky substances are hydrophobic. Talc is very suitable for binding these stickies because it has a naturally hydrophobic surface which enables it to be readily adsorbed onto sticky surfaces and to make them less tacky by coating.
Furthermore, the use of montmorillonites, such as bentonite, for controlling impurities in the paper pulp is described, for example, in U.S. Pat. No. 5,368,692. The alkali treatment of bentonites is also discussed as a possibility.
U.S. Pat. No. 4,964,955 likewise describes a process for reducing the impurities in paper production. There, a particulate composition containing (a) a water-soluble cationic polymer which is applied to (b) a substantially water-insoluble particulate substrate is used for binding impurities. The polymer should be sufficiently electropositive so that the particulate composition has a zeta potential of at least about +30 mV. The polymer is preferably a poly(dialkyldiallylammonium halide). The substrate is, for example, a phyllosilicate mineral.
In a similar manner, EP 0 760 406 A2 relates to a combination of a poly(dadmac/acrylamide) and a bentonite for binding impurities.
GB 2 297 334 A in turn discloses the use of a smectic clay for controlling impurities, the smectic clay being modified as follows: monovalent exchangeable cations are present in an equivalent ionic fraction in the range of from 0.20 to 0.60; a first type of bivalent exchangeable cations is present in an equivalent ionic fraction in the range of from 0.40 to 0.80; and a second type of bivalent exchangeable cations is present in an equivalent ionic fraction in the range of from 0.00 to 0.20, the first type of bivalent exchangeable cations comprising calcium and the second type of bivalent exchangeable cations comprising magnesium.
Many of the compositions used in the prior art for binding impurities are very expensive and not optimally suitable for certain impurity compositions. There is therefore a constant need for compositions for binding impurities in paper production.
An object of the present invention was therefore to provide an improved process for binding impurities in paper production, in which a composition which is easy and economical to prepare can be used and which permits a high degree of binding of impurities, including hydrophobic fractions.
According to one aspect of the invention, this object is achieved by the method as claimed in claim 1.
Thus, in the present invention, it was surprisingly found that surprisingly good binding of impurities in a method for binding impurities in paper production can be provided by the use of a bentonite which has a proportion of the monovalent cations, based on the cation exchange capacity (referred to herein as CEC), of at least about 0.7 (i.e. 70%), and a CEC (total) of at least 85 meq/100 g.
In the context of the present invention, impurities are understood as meaning both tacky substances, referred to in the literature as stickies, and so-called pitch, i.e. primarily tree resin components. Reference may be made here to the statements made in the introduction to the description with regard to the impurities. A detailed list of the pitch and stickies constituents is to be found, for example, in Wo01/71092 on pages 1 and 2, and the disclosure there is hereby expressly incorporated by reference into the present description.
As mentioned above, the impurities are thus primarily anionic (negatively charged) or hydrophobic. Thus, it was all the more surprising that the highly activated bentonites used according to the invention and having a high CEC can very readily bind both anionic and hydrophobic impurity fractions and can neutralize them in their harmful effects. The bentonites used according to the invention themselves have a relatively high negative layer charge and subsequently make this high (negative) surface charge available in delaminated form to the paper pulp. Thus, good binding of impurities would not be expected for anionic or hydrophobic impurities. It would also be expected that a calcium bentonite binds such impurities better because a major part of the charges of the bentonites is saturated by the calcium ions and these could immobilize impurities, for example, via soap formation and fatty acids in the tree resin. In particular, the stickies, such as tree resin particles, contain many rather nonpolar (hydrophobic) components, e.g. triglycerides. These should bind particularly well to nonpolar surfaces, such as, for example, those of talc. Talc has no surface charges and is therefore also described in the prior art as being optimum for the binding of (hydrophobic) impurities.
The results in the context of the present invention, according to which both nonpolar and anionic impurities can be efficiently bound in the method according to the invention with bentonites which make available a large surface having numerous negative charges are therefore unexpected.
The method according to the invention with the use of the special bentonite described herein can be used generally in all methods for paper or cardboard production. Accordingly, the expressions paper pulp and fiber suspension are intended generally to include all impurity-containing compositions or streams which are used in paper production. Otherwise, the expressions “pulp” and “fiber suspension” are familiar to the person skilled in the art and need not be explained in detail here.
In a preferred embodiment according to the invention, the pulp or the fiber suspension is a (fine) groundwood-containing suspension. Groundwood is in general finely digested (finely beaten wood, generally without further chemical or thermal treatment). The groundwood suspension is either used directly after comminution or is subjected to a peroxide bleach, in which case so-called peroxide-bleached groundwood forms. It has surprisingly been found that the bentonite used according to the invention gives particularly good results in the case of paper types containing groundwood or peroxide-bleached groundwood. However, the method according to the invention can also be advantageously used in the case of other paper types. Thus, for example, the pulp or fiber suspension (in addition to the groundwood) may also contain highly purified fiber fractions, as is the case, for example, in so-called newsprint paper. The invention furthermore gives very good results in the case of so-called “deinked pulp” (DIP). This is a paper stock which is produced from wastepaper. In particular, hydrophobic stickies occur there, from the stickies of magazines and newspapers. These too can be readily bound in the end product by the bentonite used according to the invention. Further so-called paper stocks in which the bentonite according to the invention can be advantageously used comprise TMP (thermomechanical pulp) sulfate pulp, sulfite pulp and mixtures of different chemical pulps. Depending on the paper type and localization of the paper mill, such chemical pulps are mixed in different ratios and adapted to the material requirements of the end product.
In an advantageous embodiment according to the invention, the preferred groundwood fraction in the paper pulp or fiber suspension is at least 10% by weight, in particular at least 30% by weight, based in each case on the dry weight of the total pulp or suspension.
The bentonite in the method according to the invention probably acts without the invention being limited to the correctness of this assumption, in that it binds the impurities or interacts with them and thus, counteracts the aggregation and deposition on the parts of the paper machine, such as, for example, the rolls.
According to the invention it is important that the bentonite used has a cation exchange capacity (CEC) of at least 85 meq/100 g, preferably at least 90 meq/100 g, in particular at least 95 meq/100 g.
“Cation exchange capacity” (CEC) is understood as meaning the sum of all exchangeable cations, stated in meq/100 g and determined by the CEC analysis method as explained below before the example section (determination of the cation exchange capacity). The cation exchange capacity thus comprises, for example, the sum of all exchangeable divalent and monovalent cations, such as calcium, magnesium, sodium, lithium and potassium ions. For the determination of the cation exchange capacity, the bentonite is treated with an ammonium chloride solution. Owing to the high affinity of the ammonium ions for the bentonite, virtually all exchangeable cations are exchanged for ammonium ions. After separation and washing, the nitrogen content of the bentonite is determined and the content of ammonium ions is calculated therefrom.
It is possible to use both natural bentonites and bentonites obtained by activation, for example from calcium bentonites, provided that the above conditions for the fraction of monovalent cations, based on the CEC, and the minimum values for the CEC are complied with. Processes for producing or activating bentonite are known per se to the person skilled in the art and need not be explained in detail here. For example, it is possible to start from a calcium bentonite having a suitable CEC and to treat it with an alkali metal carbonate, e.g. sodium carbonate. In the treatment or activation of the phyllosilicate, contact can be established in any desired manner familiar to the person skilled in the art, for example by preparing a solids mixture, a suspension with the phyllosilicate and the sodium carbonate or treatment or spraying of the phyllosilicate with a solution of the sodium carbonate.
For example, according to the first method variant, a calcium-containing crude bentonite having a water content of from about 25 to 40% by weight is kneaded with solid sodium carbonate, dried and milled. The crude bentonite is broken beforehand into pieces of less than 3 cm diameter. If the crude bentonite does not have the stated water content, this is established by spraying with water.
The activation can also be effected, for example, as follows: 350 g of crude bentonite having a water content of from about 30 to 35% by weight are introduced into a mixing apparatus (for example a Werner & Pfleiderer mixer (kneader)) and kneaded for 1 minute. The amount of sodium carbonate (soda) which corresponds to the difference between CEC and sodium content of the bentonite is then added while the mixing apparatus continues to run and further kneading is effected for 10 min. Here, the added amounts are based on the anhydrous bentonite. If required a little more distilled water is added so that the kneaded material “shears” thoroughly. The kneaded material is then comminuted in to small pieces and dried to a water content of 10±2% in a forced-circulation drying oven at about 75° C. for from 2 to 4 hours. The dry material is then milled in a rotor beater mill (e.g. in a Retsch mill) over a 0.12 mm sieve. The CEC and the fraction thereof of sodium ions were determined as described further below.
Overactivation of the bentonite, for example with soda, is likewise possible, it being possible to use more soda than would be stoichiometrically required for complete activation of the bentonite.
In a particularly preferred embodiment according to the invention, the stated fraction of monovalent cations is based on the fraction of sodium, potassium and lithium ions, in particular of sodium ions.
In a preferred embodiment according to the invention, the bentonite used has a swellability of at least 25 ml/2 9, in particular of at least 30 ml/2 g, more preferably at least 35 ml/2 g. Thus, it has surprisingly been found that bentonites having such high swellability permit particularly advantageous binding of impurities. The swelling volume is determined as follows: a calibrated 100 ml measuring cylinder is filled with 100 ml of distilled water. 2.0 g of the substance to be measured are introduced slowly in portions of from 0.1 to 0.2 g onto the water surface. After the material has sunk, the next portion is added. After the end of the addition, a waiting time of 1 hour is allowed and the volume of the swollen substance is then read in ml/2 g.
Furthermore, it has been found that the proportion of iron ions, based on the CEC, should preferably be less than about 0.005 (0.5%). It has been found that such bentonites give better results with regard to the whiteness of the paper pulp.
According to a further preferred aspect, the proportion of the monovalent cations, based on the CEC of the bentonite, is more than 0.7, in particular more than 0.8, preferably more than 0.81, more preferably more than 0.85. It is furthermore preferable if the proportion of calcium and/or magnesium ions, based on the CEC of the bentonite, is less than 0.2, in particular less than 0.18, preferably less than 0.15.
In a further preferred embodiment according to the invention, the BET surface area (determined according to DIN 66131) of the bentonites used is less than 100 m²/g, in particular less than 90 m²/g. It is surprising that bentonites having a relatively low specific BET surface area exhibit particularly advantageous binding of impurities in comparison with bentonites which can provide a higher specific surface area for adsorption of impurities.
Typically, the demand for cationic charges in the headbox decreases while the method according to the invention is being carried out. This is demonstrated by the binding of the negatively charged impurities by charge interactions.
The concentration of the impurities in paper production is typically determined in the white water by the three customary processes of cation demand (cationic charge demand), stability measurement and chemical oxygen demand. In the case of cation demand, it is assumed that the impurities are all negatively charged and the white water filters in short-chain cationic polyelectrolytes. The consumption is converted into the so-called cation demand. In the turbidity measurement, it is assumed that the impurities are partly present in colloidal form and their concentration can be determined via the extinction caused by the turbidity. In the case of the chemical oxygen demand, the proportion of organic compounds present is tested by means of an oxidizing agent. Although these methods are very widely used in the paper world, more recent investigations have shown that they average over the total ingredients in white water and only partly detect particularly critical impurities. This arises, for example, from the fact that the so-called tree resin colloids, some of which are composed of hydrophobic compounds, may carry only small surface charges and hence contribute little to the cation demand. On the other hand, lignins have a high cationic demand; if they are present in the white water, they interfere only very little in paper production. More recent investigations furthermore show that the correlation between the turbidity measurement and the concentration of colloidal impurities is not always present. Owing to this more recent experience with the customary impurity determination methods, the additives according to the invention were also characterized in their action by more recent methods. These are, for example, a gas chromatographic analysis of the white water by the method of F. Orsa and B. Holmbom “A Convenient Method for the Determination of Wood Extractives in Papermaking Process Waters and Effluents”, Journal of Pulp and Paper Science, Vol. 20 No. 12 December 1994, pp J361. In the production of a groundwood-containing paper, the individual tree resin components are determined in their concentration by a gas chromatographic method. This is a complete, quantitative analysis, whereas the standard methods of determination, such as turbidity, cation demand and chemical oxygen demand, are actually to be considered as only being semi-quantitative at best. Furthermore, L. Vähäsalo et al. (loc. cit., cf. “Flowcytometrische Analyse des Siebwassers [Flow cytometric analysis of white water]”, further below, show that so-called flow cytometry is very suitable for determining the number of colloidal impurities in paper white waters. This new method was therefore also used in the present invention in order to show the impurity-reducing effect of the bentonites according to the invention.
The addition of the bentonite used according to the invention to the pulp or fiber suspension can be effected at any desired point in the paper production which is suitable for the person skilled in the art. In particular, the addition directly in the pulper is also advisable because a long contact time with the paper stock is possible there and there is the probability of a high degree of binding of impurities. Further addition points are in the entire so-called high-consistency stock region. Addition for dissolved air-floatation for water purification is also conceivable. In many cases, an already present addition point for additives, for example in the form of a metering apparatus or metering pump, will also be present in the apparatuses used in each case for paper production, which apparatus or pump can be used for the addition of the bentonite used according to the invention. The bentonite can be used both in powder form and in the form of a suspension or slurry. The suspension or slurry will in many cases permit better meterability and is more easily automatable in industrial, continuous processes.
It has furthermore been found that the effect of the bentonite used according to the invention is particularly positive if a certain particle size is maintained. Thus, according to a particularly preferred embodiment of the invention, the particle size of the bentonite is chosen so that in the wet sieve residue less than 2% by weight, preferably less than 1% by weight, in particular less than 0.5% by weight, is 45 μm. The determination of the wet sieve residue is explained in more detail before the examples. The preferred particle size can also be determined by the light scattering method (Malvern). In a particularly preferred embodiment according to the invention, the median particle size (D50) (based on the sample volume) is from 0.5 to 10 μm, in particular from 2 to 6 μm, particularly preferably from 3 to 5 μm.
In the present invention, it was also surprisingly found that the use of the bentonite used according to the invention leads to particularly good binding of impurities if talc is not used in the method. The use of cationic polymers, such as, for example, poly(dadmac) or polyacrylamide, according to the prior art can also be reduced or even completely omitted with the aid of the bentonite used according to the invention.
The amounts of bentonite used in the method according to the invention can be determined by the person skilled in the art in a routine manner using empirical experiments. In most cases, it is advantageous to use amounts of from 0.5 to 12 kg/t of paper pulp or fiber suspension, preferably from 1 to 8 kg/t, and in particular from 1.5 to 7 kg/t, based in each case on the anhydrous pulp/suspension (dry weight).
In the present invention, it was surprisingly also found that the method according to the invention permits not only very good binding of anionic impurity fractions, such as fatty acids, but also outstanding binding or elimination of hydrophobic impurity fractions, such as sterols, steryl esters and triglycerides. The results achieved here surprisingly surpass both those which were obtained with conventional bentonites and those of talc.
A further aspect of the present invention relates to the use of a bentonite as described herein for binding impurities in paper production. As mentioned above, the bentonite is preferably used in a paper pulp or fiber suspension which contains groundwood fractions. However, all paper types or pulps are covered by the use according to the invention. The paper types mentioned further above, such as paper types containing groundwood or peroxide-treated groundwood, those which (in addition to the groundwood) also contain highly purified fiber fractions, as is the case, for example, in so-called newsprint paper, so-called deinked pulp (DIP), TMP (thermomechanical pulp), sulfate pulp, sulfite pulp and mixtures of different chemical pulps are particularly preferred.
Method section: Unless stated otherwise, the analytical methods stated below are used:
1. Determination of the Cation Exchange Capacity (CEC Analysis) and of the Cation Fractions
Principle: The clay is treated with a large excess of aqueous NH₄Cl solution and washed out, and the amount of NH₄ ⁺ remaining on the clay is determined according to Kjeldahl.
Me⁺(clay)⁻+NH₄ ⁺—NH₄ ⁺(clay)⁻+Me⁺
(Me⁺═H⁺, K⁺, Na⁺, ½Ca²⁺, ½Mg²⁺. . . )
Apparatuses: Sieve, 63 μm; conical flask with ground glass joint, 300 ml; analytical balance; membrane suction filter, 400 ml; cellulose nitrate filter, 0.15 μm (from Sartorius); drying oven; reflux condenser; hotplate; distillation unit, VAPODEST-5 (from Gerhardt, No. 6550); graduated flask, 250 ml; flame AAS.
Chemicals: 2N NH₄Cl solution, Nessler's reagent (from Merck, Art. No. 9028); boric acid solution, 2% strength; sodium hydroxide solution, 32% strength; 0.1 N hydrochloric acid; NaCl solution, 0.1% strength; KCl solution, 0.1% strength.
Procedure: 5 g of clay are sieved through a 63 μm sieve and dried at 110° C. Thereafter, exactly 2 g are weighed into the conical flask having a ground glass joint on the analytical balance by differential weighing, and 100 ml of 2N NH₄Cl solution are added. The suspension is boiled under reflux for one hour. In the case of bentonites having a high CaCo₃content, ammonia may be evolved. In these cases, NH₄Cl solution must be added until the odor of ammonia is no longer perceptible. An additional check can be carried out with a moist indicator paper. After a standing time of about 16 h, the NH₄ ⁺-bentonite is filtered off over a membrane suction filter and washed with demineralized water (about 800 ml) until substantially free of ions. The testing of the wash water for freedom from ions is carried out for NH₄ ⁺ ions with Nessler's reagent which is sensitive to them. The washing time can vary from 30 minutes to 3 days, depending on the type of clay. The washed-out NH₄ ⁺-bentonite is removed from the filter, dried at 110° C. for 2 h, milled, sieved (63 μm sieve) and dried again at 110° C. for 2 h. Thereafter, the NH₄ ⁺ content of the bentonite is determined according to Kjeldahl.
Calculation of the CEC: the CEC of the clay is the NH₄ ⁺ content of the NH₄ ⁺ bentonite, determined by means of Kjeldahl (for CEC of some clay minerals, cf. appendix). The data are given in meq/100 g of clay.
Example: Nitrogen content=0.93%;
Molecular weight: N=14.0067 g/mol
$C E C = \frac{0.93 \times 1000}{14.0067} = 66.4 meq / 100 g$
CEC=66.4 meq/100 g of NH₄ ⁺-bentonite
Exchanged Cations and the Proportions Thereof:
The cations liberated by the exchange are present in the wash water (filtrate). The proportion and the type of the monovalent cations (“exchanged cations”) are determined spectroscopically in the filtrate according to DIN 38406, part 22. For example, for the AAS determination, the wash water (filtrate) is concentrated, transferred to a 250 ml graduated flask and made up to the mark with demineralized water. Suitable measuring conditions for FAAS are shown in the following tables.


Element	Calcium	Potassium	Lithium	Magnesium	Sodium

Wavelength	422.7	766.5	670.8	285.2	589.0
(nm)				(202.6)
Gap width	0.2	0.5	0.5	0.5	0.2
(nm):
Integration	3	3	3	3	3
time (sec):
Flame gasses:	N₂O/C₂H₂	Air/C₂H₂	Air/C₂H₂	N₂O/C₂H₂	Air/C₂H₂
Background	no	no	no	yes	no
comp.:
Measuring	conc.	conc.	conc.	conc.	conc.
method:
Ionization	0.1% KCI	0.1% NaCl	0.1% NaCl	0.1% KCI	0.1% KCI
buffer:
Burner	15-20°	—	—	—	—
position
Calibration	1-5 mg/l	1-5 mg/l	2-10 mg/l	0.5-3 mg/l	1-5 mg/l
standard				(5-40 mg/l)
(mg/l):


Element	Aluminum	Iron

Wavelength	309.3	248.3
(nm):
Gap width	0.5	0.2
(nm):
Integration time (sec):	3	3
Flame gasses:	N₂O/C₂H₂	Air/C₂H₂
Background comp.:	yes	no
Measuring method:	conc.	conc.
Ionization buffer:	0.1% KCl	—
Burner position	—	—
Calibration	10-50 mg/l	1-5 mg/l
standard(mg/l):

Calculation of the Cations:
$Me = \frac{Me - value (mg / 1) \times 100 \times dilution}{4 \times weight taken (in g) \times molar mass (g / mol)} = meq / 100 g$
Molar mass (g/mol): Ca=20.040; K=39.096; Li=6.94; Mg=12.156; Na=22.990; Al=8.994; Fe=18.616
In the case of so-called overactivated bentonites, i.e. those which were activated with an amount of, for example, sodium carbonate which is greater than the stoichiometric amount, the sum of the amounts of monovalent cations determined may be greater than the CEC determined as stated above. In such cases, the total content of monovalent cations (Li, K, Na) is regarded as 100% of the CEC.
The invention is now illustrated in more detail with reference to the following, non-limiting examples.
2. Determination of the BET Surface Area:
The determination was effected according to DIN 66131 (multipoint measurement).
3. Determination of the Wet Sieve Residue:
With the use of pigments and fillers, it is of interest whether the material to be investigated contains coarse fractions which differ in their particle size from the normal particles and how much of said coarse fractions said material contains. These fractions are determined by sieving an aqueous suspension with water as wash liquid. The wet sieve residue is considered to be the residue determined under specified conditions.
Apparatuses: analytical balance, plastic beaker, Pendraulik LD 50; sieve: 200 mm diameter, mesh size 0.025 (25 μm), 0.045 mm (45 μm), 0.053 mm (53 μm) or 0.063 mm (63 μm); ultrasonic bath.
First, a 5% strength suspension of the bentonite (oven dry, i.e. after drying at 110° C.) in 2000 g of water was prepared. For this purpose, the bentonite is stirred in at 930 rpm in about 5 min. After a stirring time of a further 15 min at 1865 rpm, the suspension is poured into the cleaned and dried sieve (mesh size 45 μm) and washed with flowing tap water while tapping until the wash water runs out clear. After the washing of the sieve residue with tap water, the sieve is placed for 5 min in an ultrasonic bath in order to sieve off the remaining fine fractions. When using the sieve in the ultrasonic bath, it should be ensured that no air remains between water surface and sieve bottom. After the ultrasonic treatment, rinse again briefly with tap water. Thereafter, the sieve is removed and the water in the ultrasonic bath is replaced. The procedure in the ultrasonic bath is repeated until contamination of the water is no longer detectable. The sieve with the remaining residue is dried to constant weight (oven dry) in a forced-circulation drying oven. After cooling, the residue is transferred by means of a brush into a dish. Evaluation: wet sieve residue (WSR) in (%), based on the amount weighed out.
4. Particle Size Determination According to Malvern:
This is a customary method. A Mastersizer from Malvern Instruments Ltd, UK, was used according to the manufacturer's instructions. The measurements were carried out with the sample chamber provided (“dry powder feeder”) in air and the values based on the sample volume were determined.
5. Investigation of Binding of Impurities: in the Investigation of the Binding of Impurities, the Following Procedure was Adopted:
a) Preparation of Paper Stock and Filtration:
The chosen paper stock (e.g. 45% of chemical pulp and 55% of peroxide-bleached groundwood) can either be obtained directly from the paper mill or stored in a refrigerator before use. The paper stock was then thoroughly shaken at 20 g absolutely dry and diluted to 2% with warm demineralized water in a 2000 ml beaker. While being stirred at 400 rpm, the paper stock batch warmed up to 40° C. with the aid of a hotplate. When the temperature is reached, the amount of adsorbent to be tested is added to the paper stock batch with the aid of a Pasteur pipette. Thereafter, the adsorption time in the stock batch is fixed at 30 min at 40° C. and the mixture is stirred for this time at 400 rpm. Thereafter, the paper stock batch of the adsorbent is diluted to 1% solids content with the aid of demineralized water (40° C.).
For the white water preparation, 1000 g of this dilute stock batch (1% by weight solids content) is drained in a drainage and retention apparatus (Mütek DF3 03 from Mütek, Germany) for 420 seconds (170 μm sieve, stirring speed 700 rpm). The white water samples are investigated analytically.
b) Flow Cytometric Analysis of the White Water:
Here, so-called flow cytometry was used, as described in Vähäsalo et al., “Use of Flow Cytometry In Wet End Research”, Paper Technology, 44 (1), page 45, February 2003 and additionally in “Effects Of pH and calcium chloride on pitch in peroxide-bleached mechanical pulp suspensions”, 7th European Workshop on Lignocellulosics and Pulp, Aug. 26-29, 2002, Åbo/Finland. In brief, a light scattering method for counting the particles is combined with fluorescence marking.
c) Gas Chromatographic Analysis of the White Water:
Here, the method of F. Orsa and B. Holmbom “A Convenient Method for the Determination of Wood Extractives in Papermaking Process Waters and Effluents”, Journal of Pulp and Paper Science, Vol. 20 No. 12, December 1994, pp J361, was used.
The following was found:
FIG. 1 shows a graph of the dependence of the concentration of the impurity particles in the white water (filtrate water) on the type and amount of the adsorbent used (bentonite or talc).
The invention is now illustrated further with reference to the non-limiting examples below.

EXAMPLE 1

The following materials were investigated for the binding of impurities.
1. Calcium Bentonite (Bentonite 1)
The analytical data of the calcium bentonite used are summarized in table 1. The proportions and the CEC are to be found in table 2.

TABLE 1

Analytical data of the calcium bentonite (bentonite 1)

Water content

12.2%

by weight

	pH (5% by weight of suspension	9.0
	in water)

Montmorillonite content	100%	by weight
(methylene blue method)
Quartz	0.5%	by weight
Calcite	<1%	by weight
Specific surface area (BET)	86	m²/g

The wet sieve residue (45 μm) was less than 0.5% by weight.

TABLE 2

Proportion of the monovalent cations, based on the cation
exchange capacity (CEC) of the calcium bentonite (bentonite
1); CEC (total) = 106 meg/100 g.

	Cation	Proportion of the CEC (%)

	Na	34
	K	2
	Li	0

2. Cationized Talc (Product Malusil 75-7 K from Talc de Luzenac)
3. Bentonite According to the Invention (Bentonite 2)
Bentonite 2 was obtained from bentonite 1 by kneading bentonite 1 with 5% by weight of sodium carbonate, based on the anhydrous bentonite, according to the above method, drying to a water content of 10% by weight and milling to a particle size corresponding to that of bentonite 1 (comparison: table 2). By means of these processing steps, the mineralogical data of the bentonite are not changed, so that the montmorillonite content and the content of impurity minerals remain unchanged. The BET surface area was 85±2 m²/g.
The analytical data for bentonite 2 are shown in table 3.

TABLE 3

Proportion of the monovalent cations, based on the cation exchange
capacity (CEC) of the bentonite according to the invention (bentonite
2); CEC (total) = 102 meq/100 g of bentonite.

	Cation	Proportion of the CEC (%)

	Na	98
	K	2
	Li	0

Using the two bentonites 1 and 2, the binding of impurities was investigated as described in the method section. For carrying out the filtration experiments, a paper stock which was taken from a paper machine and consisted of 45% of long fiber chemical pulp and 55% of peroxide-bleached groundwood was used.
For comparison, in each case a “zero sample” was run, i.e. no adsorbents were used for binding impurities.
For characterization of the filtrate waters (white waters) with regard to a reduction of impurities, the abovementioned flow cytometry was used. The results are shown in FIG. 1. The amount of adsorbent used (bentonite or talc) is plotted against the concentration of the impurity particles in the white water. It is clearly found that the bentonite 2 according to the invention exhibits substantially better binding of impurities than bentonite 1 or talc even when a small amount of three kilograms per tonne, based on the paper pulp/suspension, is used in dry pulp.
The content of fatty acids, lignins, sterols, steryl esters and triglycerides was determined for the above samples by means of the gas chromatographic analysis (cf. method section). The bentonites 1 and 2 were used in each case in an amount of 6 kg/t of paper (dry weight); the cationized talc was used in an amount of 11.25 kg/t of paper, since 6 kg/t gives poor results. The values obtained are shown in table 4.

TABLE 4

Concentrations of individual impurities after the treatment with the
impurity-binding agents in mg/l according to gas chromatography

	Fatty			Steryl
Sample	acids	Lignins	Sterols	esters	Triglycerides

“Zero	0.08	4.38	0.25	1.26	1.69
sample”
Cat. talc	0.04	4.23	0.20	1.07	1.77
Bentonite 1	0.05	4.16	0.07	0.46	0.96
Bentonite 2	0.04	4.07	0.06	0.31	0.67

As is evident in table 4, the sample treated with the bentonite 2 according to the invention shows substantially better binding/removal of fatty acids, lignins, styrenes, styryl esters and triglycerides both compared with the sample treated with cationized talc and with the sample treated with the calcium bentonite (bentonite 1) not according to the invention.
In a further example, the bentonite according to the invention was compared with conventional bentonites which have a proportion of monovalent cations, based on the CEC, of at least 0.7 (70%) but a CEC of less than 85 meq/100 g.
Once again, substantially better binding of impurities by the bentonite according to the invention was found in comparison with the conventional bentonites, even when small amounts were used.

Claims

1. A method for binding impurities in paper production, comprising the following steps:

a) providing bentonite, with a proportion of monovalent cations of at least about 0.7 based on the cation exchange capacity (CEC) of the bentonite, wherein the CEC is greater than 85 meq/100 g;

b) adding the bentonite to a paper pulp or fiber suspension; and

c) binding impurities to the bentonite in the pulp or fiber suspension.

2. The method as claimed in claim 1, characterized in that the proportion of the monovalent cations, based on the CEC of the bentonite, is more than 0.8.

3. The method as claimed in claim 1, characterized in that the proportion of calcium and/or magnesium ions in the bentonite, based on the CEC of the bentonite, is less than 0.2.

4. The method as claimed in claim 1, characterized in that the particle size of the bentonite is chosen so that in the wet sieve residue less than 2% by weight, is 45 μm.

5. The method as claimed in claim 1, characterized in that the monovalent cations in the bentonite are selected from the group consisting of sodium, potassium, lithium and mixtures thereof.

6. The method as claimed in claim 1, characterized in that the bentonite is present in particulate form having a median particle size (D50, based on volume) of from 0.5 to 10 μm.

7. The method as claimed in claim 1, characterized in that the addition of the bentonite is effected in the absence of talc.

8. The method as claimed in claim 1, characterized in that the bentonite has a swellability of at least 25 ml/2 g.

9. The method as claimed in claim 1, characterized in that the bentonite has a proportion of iron ions, based on the CEC, of less than about 0.005.

10. The method as claimed in claim 1, characterized in that the bentonite has a BET surface area of less than 100 m²/g.

11. The method as claimed in claim 1, characterized in that from about 0.5 to 10 kg of bentonite are added per tonne of paper pulp or fiber suspension (dry weight).

12. The method as claimed in claim 1, characterized in that the paper pulp or fiber suspension contains a groundwood fraction.

13. The method as claimed in claim 12, characterized in that the groundwood fraction in the paper pulp or the fiber suspension is at least 10% by weight, based on the total pulp or fiber suspension (dry weight).

14. The method as claimed in claim 1, characterized in that no additional talc is added to the paper pulp or fiber suspension.

15. (canceled)

16. (canceled)

17. (canceled)

18. (canceled)

19. The method as claimed in claim 1, wherein the CEC of the bentonite is greater than 95 meq/100 g.

20. The method as claimed in claim 1 characterized in that the proportion of the monovalent cations, based on the CEC of the bentonite, is more than 0.85.

21. The method as claimed in claim 1 characterized in that the proportion of calcium and/or magnesium ions in the bentonite, based on the CEC of the bentonite, is less than 0.15.

22. The method as claimed in claim 1 characterized in that the particle size of the bentonite is chosen so that the wet sieve residue less than 2% by weight is 45 μm.

23. The method as claimed in claim 1 characterized in that the bentonite is present in particulate form having a medium particle size (D50, based on volume) of from 2 to 6 μm.

24. A method for binding hydrophobic impurities in paper production, comprising the following steps:

b) adding the bentonite to a paper, pulp or fiber suspension; and

c) binding hydrophobic impurities to the bentonite in the pulp and fiber suspension.